Method for improving mouldability of magnesium-alloy sheet material, and magnesium-alloy sheet material produced thereby

ABSTRACT

A method for increasing formability of magnesium alloy sheet and the magnesium alloy sheet prepared by the same are provided, in which the method includes the following steps: forming {10-12} twins in magnesium alloy sheet (step 1); and annealing the magnesium alloy sheet of step 1 (step 2). The present invention also provides the magnesium alloy sheet containing {10-12} twins prepared by the method. Accordingly, the room temperature formability and the warm formability are increased by forming {10-12} twins through deformation on the magnesium alloy sheet and subsequently performing annealing, because it is possible to artificially form {10-12} twins without increasing dislocations in the magnesium alloy sheet and accommodate the deformation generated during the forming via annihilation of the twins.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of International Application No. PCT/KR2012/010780, filed Dec. 12, 2012, the entire disclosure of which is incorporated herein by reference in its entirety.

FIELD

The present invention relates to a method for increasing formability of magnesium alloy sheet and the magnesium alloy sheet prepared by the same method.

DESCRIPTION OF RELATED ART

Magnesium is the lightest metal whose density is 1.74 g/cm³, the smallest density among all of structural metals including aluminum and steel. Magnesium is also characterized by high specific strength, excellent machinability, dampling capacity, and electromagnetic wave shielding property.

For the properties of magnesium alloys as those mentioned above, use of magnesium alloys has recently been growing, replacing steel and aluminum alloys, to suit demands for lighter weighted transport equipments with higher fuel efficiency. The magnesium alloys also find increasing use in applications like mobile phones or laptop computers where LTSS (light, thin, short and small), excellent electromagnetic wave shieldability, etc. are required. However, magnesium alloy has problems of comparatively lower strength, ductility, and corrosion resistance than those of aluminum alloy. Ductility of magnesium alloy can be improved by raising process temperature. However, due to difficulty of compression-molding products with complicated shapes or shaft corners, commercialization of magnesium alloy is still restricted.

Conventionally, magnesium alloy parts have been prepared largely by die casting or squeeze casting. However, the parts prepared by the said conventional methods demonstrated weakness in mechanical properties including elongation and strength mostly because of casting defects, for example gas cavity, compared with the other parts prepared by plastic working including rolling, extruding, forging, etc.

In general, magnesium alloy has the hexagonal closed packed structure (HCP), suggesting that it has less slip systems than other metals which results in the low formability at room temperature.

Particularly, according to von Mises yield criterion, 5 independent slip systems must operate to deform magnesium alloy randomly. However, because the critical resolved shear stress (CRSS) of non-basal slip for magnesium and its alloys is much larger than that of basal slip at room temperature, it is difficult for non-basal slip to operate properly. Accordingly, only 2 independent basal slips operate at room temperature, resulting in the low formability.

It is generally known that wrought magnesium alloys prepared by the conventional method in the art develop strong texture. That is, the wrought magnesium alloys have most basal planes of crystal grains aligned parallel to the rolling plane of the sheet or extrusion direction of the extruded bar, so that when subjected to tensile stress applied in the direction parallel to processing direction (i.e., rolling direction or extrusion direction), Schmid factor of the basal slip approaches 0, and the basal slip, which is the main slip system of magnesium alloy, is hardly activated.

According to the conventional method, the wrought magnesium alloys are warm formed at 200° C. at which the non-basal slip is activated, in order to improve formability of the wrought magnesium alloy.

However, the warm forming has following disadvantages compared with the cold forming performed at room temperature; an equipment to control molding temperature is additionally required; and the production cost goes high because of the extended molding time. In addition, die soldering between mold and magnesium alloy and the difficulty in regulation of thermal strain of mold are also known to be the problems of warm forming.

Following methods have been informed as the way to improve formability of wrought magnesium alloys.

Korean Patent No. 10-0783918 (Registration date: Dec. 3, 2007) relates to the method to improve formability of magnesium alloy sheet at room temperature by the texture control, in which slab-shaped magnesium ingot in mushy state prepared from magnesium alloy sheet by using twin roll casting undergoes 6-step hot-rolling to control texture and improve formability at room temperature (patent reference 1). However, KR10-0783918 is limited to the use of magnesium alloy sheet twin roll casting, indicating the limitation of applicability and problem of difficulty in controlling the complicated rolling condition.

Korean Patent No. 10-0860091 (Registration date: Sep. 18, 2008) relates to magnesium alloy with reduced axial ratio and the method for preparing the said magnesium alloy sheet, in which rare earth elements or commercialized alloys composed of rare earth elements are added to magnesium alloy to reduce the ratio of unit cell height (c) to unit cell side length (a) of hexagonal closed packed structure in order to prepare magnesium alloy with improved formability at room temperature (patent reference 2). However, the magnesium alloy prepared as suggested in patent reference 2 has a problem of difficulty of mass-production because production cost inevitably increased owing to need for the addition of the expensive rare earth elements.

In the course of study to improve formability of wrought magnesium alloy, i.e., the magnesium alloy sheet, at room temperature, the present inventors found out that the formability of magnesium alloy sheet at room temperature or at warm-forming condition could be improved, by deforming magnesium alloy sheet to develop {10-12} twins and removing the dislocations increased by the deformation by annealing, that is, by forming {10-12} twins in magnesium alloy without increasing dislocations and accommodating the deformation induced during forming via the {10-12} twin annihilation, and thus completed the present invention.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for increasing formability of magnesium alloy sheet.

It is another object of the present invention to provide a magnesium alloy sheet containing {10-12} twins prepared by the above method.

To achieve the above objects, the present invention provides a method for increasing formability of magnesium alloy sheet comprising the following steps:

forming {10-12} twins in magnesium alloy sheet (step 1); and

annealing the magnesium alloy sheet of step 1 (step 2).

The present invention also provides a magnesium alloy sheet containing {10-12} twins prepared by the said method.

As explained hereinbefore, the method for increasing formability of magnesium alloy sheet of the present invention is characterized by forming {10-12} twins through the deformation of the sheet which may be accomplished by compressive deformation in the direction parallel to the basal plane of texture of the sheet and removing, by annealing, dislocations increased due to the deformation. Accordingly, the method provides advantageous effect of improved formability of magnesium alloy sheet at room temperature or at warm forming condition, by artificially forming {10-12} twins without increasing dislocations in magnesium alloy, and by easily accommodating the deformation induced during forming via twin annihilation.

BRIEF DESCRIPTION OF THE DRAWINGS

The application of the preferred embodiments of the present invention is best understood with reference to the accompanying drawings, wherein:

FIG. 1 is a set of photomicrographs illustrating the changes of microstructure according to compressive deformation in the magnesium alloy sheet of the present invention;

FIG. 2 is a graph illustrating twin volume fractions according to the compressive deformation in the magnesium alloy sheet of the present invention;

FIG. 3 is a set of photographs illustrating the results of electron back scattered diffraction (EBSD) displaying the changes of texture according to compressive deformation in the magnesium alloy sheet of the present invention;

FIG. 4 is a diagram illustrating the testing device for Erichsen test;

FIG. 5 is a diagram illustrating the specimen used for evaluating formability in Erichsen test;

FIG. 6 is a set of photographs illustrating the shape of the specimen used for Erichsen test of the magnesium alloy sheet of the present invention;

FIG. 7 is a load-deflection graph measured during Erichsen test with the magnesium alloy sheet of the present invention (Example 2);

FIG. 8 is a load-deflection graph measured during Erichsen test with the magnesium alloy sheet of the present invention (Comparative Example 1);

FIG. 9 is a photograph illustrating the result of EBSD displaying the changes of microstructure in the magnesium alloy sheet of the present invention (Example 2);

FIG. 10 is a photograph illustrating the result of EBSD displaying the changes of microstructure in the magnesium alloy sheet of the present invention over the forming (Example 2);

FIG. 11 is a photograph illustrating the result of EBSD displaying the changes of microstructure in the magnesium alloy sheet of the present invention over the forming (Example 2);

FIG. 12 is a photograph illustrating the result of EBSD displaying the changes of microstructure in the magnesium alloy sheet of the present invention (Comparative Example 1); and

FIG. 13 is a photograph illustrating the result of EBSD displaying the changes of microstructure in the magnesium alloy sheet of the present invention over the forming (Comparative Example 1).

DESCRIPTION OF THE PREFERRED EMBODIMENTS As used herein, “A˜B” is defined as a range between at least A (A or more) and up to B (B or under), unless defined otherwise.

As used herein, “room temperature” means the temperature range of 0˜50° C.

The present invention provides a method for increasing formability of magnesium alloy sheet comprising the following steps:

forming {10-12} twins in magnesium alloy sheet (step 1); and

annealing the magnesium alloy sheet of step 1 (step 2).

The present invention also provides a magnesium alloy sheet comprising {10-12} twins prepared by the said method.

Hereinafter, the present invention is described in detail.

Before explaining the invention in detail, the formability of the magnesium alloy sheet of the present invention will be explained below based on the following principals.

In the course of rolling or extruding, an intense texture having a preferred crystal orientation to a specific direction is formed on the processed magnesium alloy material. In the rolled magnesium alloy material, the basal planes of crystal grains are aligned parallel to the rolling direction. In the extruded magnesium alloy material, the basal planes of crystal grains are aligned parallel to the extrusion direction.

When magnesium alloy sheet is formed by stretch processing or deep drawing processing, a multi-axial tensile strain is developed parallel to the rolling plane and a compressive deformation is developed along the thickness direction which is perpendicular to the rolling plane.

In general, the basal planes of crystal grains in magnesium alloy sheet are aligned parallel to the rolling plane of the magnesium alloy sheet. So, the tensile strain parallel to the rolling plane is accommodated by basal slip having the slip direction to a-axis, making cold forming comparatively easy. However, because the compressive deformation developed along the thickness direction which is perpendicular to the rolling plane requires the strain to the direction of c-axis, cold forming is difficult because of limitation in deformation to the direction of thickness caused by unaquirable deformation mode at room temperature.

Therefore, according to the present invention, {10-12} twins were artificially formed to control the texture in the magnesium alloy sheet, to improve the formability of magnesium alloy sheet at room temperature.

Unlike dislocations, the {10-12} twins are generated at a specific stress direction and the band has the angle of approximately 86° to the initial grain. The {10-12} twins are only generated when such tensile stress is loaded to the c-axis and not generated when such compressive stress is loaded to the c-axis.

Precisely speaking, in the wrought magnesium alloy in which the basal planes of crystal grains are aligned parallel to the rolling plane or the extrusion direction, when a compressive stress is loaded to the direction of processing, the magnesium alloy becomes stress state in which the c-axis of crystal grain is under tension, and thus {10-12} twins are easily generated. On the other hand, when a tensile stress is applied to the direction of extruding or rolling, {10-12} twins are hardly generated.

Also, the {10-12} twins can accommodate deformation easily during the generation and annihilation, thereby leading to the low yield strength and low strain hardening rate. The {10-12} twins are annihilated when the stress is applied to the opposite direction of the stress applied in order to form the twins.

Based on the above principles, the present invention is described in more detail.

The present invention provides a method for increasing formability of magnesium alloy sheet comprising the following steps:

forming {10-12} twins in magnesium alloy sheet (step 1); and

annealing the magnesium alloy sheet of step 1 (step 2).

In the method of the present invention, step 1 is to form {10-12} twins in the magnesium alloy sheet.

The magnesium alloy sheet as used herein can be selected from the group consisting of ZM21, ZC63, AZ91, AZ91D, AM50A, AM608, AZ31, and AZ80, but not always limited thereto and any commercial magnesium alloy sheet acceptable by those skilled in the art can be used herein without limitation.

In the method for increasing formability of magnesium alloy sheet of the present invention, the {10-12} twins can be formed in the magnesium alloy sheet by applying a compressive deformation thereto. The compressive deformation herein can be applied in the direction parallel to the rolling plane of the magnesium alloy sheet.

As explained hereinbefore, {10-12} twins can be formed by applying a compressive deformation to the direction parallel to the rolling plane of the magnesium alloy sheet, but not always limited thereto. Accordingly, any method possibly used for forming {10-12} twins can be implemented.

For the compressive deformation, it is preferred to make deformation in the range of 1˜15% to form {10-12} twins and more preferably in the range of 3˜10%.

Under loading of the less than 1% compressive deformation, it is difficult to form {10-12} twins, suggesting that the formability of magnesium alloy sheet is hardly improved. Under loading of more than 15% compressive deformation, {10-12} twins are saturated during the magnesium alloy processing, suggesting that the formability is not improved and instead {10-11} twins are formed rather to reduce the formability of magnesium alloy.

In the method of the present invention, step 2 is to treat the magnesium alloy sheet of step 1 by annealing.

The dislocations generated in the magnesium alloy sheet by the compressive deformation of step 1 can be eliminated through the annealing in step 2.

In the method for increasing formability of magnesium alloy sheet of the present invention, the annealing is preferably performed at 200˜550° C. for 10˜480 minutes, and more preferably at 350˜500° C. for 60˜240 minutes.

When the annealing is performed at the temperature under 200° C. for less than 10 minutes, the dislocations generated by the compressive deformation of step 1 are not completely eliminated, resulting in the decrease of formability of magnesium alloy sheet. When the annealing is performed at the temperature over 550° C. for more than 480 minutes, the partial dissolution occurs in the segregate or second phase of the magnesium alloy sheet, also resulting in the decrease of formability of magnesium alloy sheet.

Practical and presently preferred embodiments of the present invention are illustrative as shown in the following examples.

However, it will be appreciated that those skilled in the art, on consideration of this disclosure, may make modifications and improvements within the spirit and scope of the present invention.

In the following examples, {10-12} twins were formed under loading of a compressive deformation to the rolling direction of the magnesium alloy sheet of the invention.

EXAMPLE 1

After loading of compressive deformation on the rolled AZ31 magnesium alloy sheet (thickness: 50 mm, composition: 3.6% aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other inevitable impurities) to the rolling direction at the amount of deformation of 2%, annealing followed at 350° C. for 60 minutes.

EXAMPLE 2

After loading of compressive deformation on the rolled AZ31 magnesium alloy sheet (thickness: 50 mm, composition: 3.6% aluminum, 1.0% zinc , 0.5% manganese, magnesium, and other inevitable impurities) to the rolling direction at the amount of deformation of 5%, annealing followed at 350° C. for 60 minutes.

EXAMPLE 3

After loading of compressive deformation on the rolled AZ31 magnesium alloy sheet (thickness: 50 mm, composition: 3.6% aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other inevitable impurities) to the rolling direction at the amount of deformation of 8%, annealing followed at 350° C. for 60 minutes.

EXAMPLE 4

After loading of compressive deformation on the rolled AZ31 magnesium alloy sheet (thickness: 50 mm, composition: 3.6% aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other inevitable impurities) to the rolling direction at the amount of deformation of 8%, annealing followed at 250° C. for 60 minutes.

EXAMPLE 5

After loading of compressive deformation on the rolled AZ31 magnesium alloy sheet (thickness: 50 mm, composition: 3.6% aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other inevitable impurities) to the rolling direction at the amount of deformation of 8%, annealing followed at 450° C. for 60 minutes.

COMPARATIVE EXAMPLE 1

Neither the compressive deformation loading nor annealing was performed for the rolled AZ31 magnesium alloy sheet (thickness: 50 mm, composition: 3.6% aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other inevitable impurities).

COMPARATIVE EXAMPLE 3

Compressive deformation was not loaded on the rolled AZ31 magnesium alloy sheet (thickness: 50 mm, composition: 3.6% aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other inevitable impurities), but annealing was performed at 350° C. for 60 minutes.

COMPARATIVE EXAMPLE 4

Compressive deformation was not loaded on rolled AZ31 magnesium alloy sheet (thickness: 50 mm, composition: 3.6% aluminum, 1.0% zinc ( ) 0.5% manganese, magnesium, and other inevitable impurities), but annealing was performed at 450° C. for 60 minutes.

COMPARATIVE EXAMPLE 5

After loading of compressive deformation on the rolled AZ31 magnesium alloy sheet (thickness: 50 mm, composition: 3.6% aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other inevitable impurities) to the rolling direction at the amount of deformation of 2%, annealing was not performed.

COMPARATIVE EXAMPLE 6

After loading of compressive deformation on the rolled AZ31 magnesium alloy sheet (thickness: 50 mm, composition: 3.6% aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other inevitable impurities) to the rolling direction at the amount of deformation of 5%, annealing was not performed.

COMPARATIVE EXAMPLE 7

After loading of compressive deformation on the rolled AZ31 magnesium alloy sheet (thickness: 50 mm, composition: 3.6% aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other inevitable impurities) to the rolling direction at the amount of deformation of 8%, annealing was not performed.

Analysis

1. Analysis of Changes in Microstructure of Magnesium Alloy Sheet According to Compressive Deformation

To investigative the changes in microstructure of the magnesium alloy sheet of the present invention according to amount of compressive deformation, observation of the microstructure before and after the compressive deformation was performed. The results are shown in FIG. 1 and FIG. 2 (FIGS. 1 and 2 : Comparative Example 1, Comparative Examples 5˜7)

FIG. 1 illustrates the formation of twins, in which the light region indirectly indicates the formation of twins. FIG. 2 is a graph illustrating twin volume fractions according to the compressive deformation in the magnesium alloy sheet of the present invention.

Referring to FIG. 1 and FIG. 2, twins were not found in the initial magnesium alloy sheet before loading of the compressive deformation, but twins were increased according to the increase of the compressive deformation.

2. Analysis of Texture of Magnesium Alloy Sheet According to Compressive Deformation

To analyze the texture of the magnesium alloy sheet of the present invention according to compressive deformation, electron back scattered diffraction (EBSD) was performed before and after the compressive deformation, and the results are shown in FIG. 3 (FIG. 3 : Comparative Example 1, Comparative Examples 5˜7).

The twinned region formed by compressive deformation was reoriented at the angle of approximately 86° to the initial crystal grain. As shown in the (0002) pole figure in FIG. 3, the basal planes of crystal grains of the initial magnesium alloy sheet without compressive deformation were aligned parallel to the rolling plane. In the meantime, in the sheet with compressive deformation, the original texture was weakened because of the reorientation caused by twins and the basal planes of crystal grains were aligned perpendicular to the rolling direction. Such changes in the texture grew bigger as the compressive deformation increased.

From the above results, it was confirmed that the compressive deformation parallel to the rolling plane induced the formation of twins in the magnesium alloy sheet, by which the texture of the alloy sheet was changed.

EXPERIMENTAL EXAMPLE 1 Analysis of Formability of Magnesium Alloy Sheet According to Compressive Deformation and Heat Treatment 1

To analyze the formability of the magnesium alloy sheet of the present invention, Erichsen test was performed as shown in FIG. 4 and the results are presented in Table 1 and FIG. 6 (FIG. 6 : Comparative Example 1, Examples 2 and 3).

Particularly, the magnesium alloy sheet of the invention was prepared in the processed specimens as shown in FIG. 5 (thickness: 1 mm, diameter: 50 mm) for the Erichsen test. The Erichsen test was conducted with the specimens at 23° C. (room temperature), 100° C., 200° C., and 300° C. at the speed of 0.1 mm/s to obtain limit dome height (LDH). The presented LDH is the mean value obtained from the experiment repeated at least three times for each condition. At this time, as the LDH is bigger, the formability becomes excellent.

TABLE 1 Com- pressive Annealing strain Temp Time Limit Dome Height (%) (° C.) (Min) 23° C. 100° C. 200° C. 300° C. Example 1 2 350 60 3.2 4.2 4.4 6.6 Example 2 5 350 60 5.1 5.8 6.0 6.8 Example 3 8 350 60 6.3 7.1 8.1 8.8 Example 4 8 250 60 3.9 — — — Example 5 8 450 60 6.4 — — — Comparative — — — 3.1 3.8 4.1 5.1 Example 1 Comparative — 250 60 3.1 — — — Example 2 Comparative — 350 60 3.0 — — — Example 3 Comparative — 450 60 3.2 — — — Example 4 Comparative 2 — — 2.9 — — — Example 5 Comparative 5 — — 3.0 — — — Example 6 Comparative 8 — — 2.9 — — — Example 7

Referring to Table 1, it was suggested that the improvement in the formability was hardly observed in the magnesium alloy sheet treated with compressive deformation (or annealing) only, compared with the formability of the conventional magnesium alloy sheet which is not processed by any of the said processes (Comparative Example 1 in Table 1).

Precisely, when the magnesium alloy sheet was subjected to the compressive deformation only, the formability was not increased because the dislocations generated by the compressive deformation reduced elongation of the sheet even with the presence of twins. When the magnesium alloy sheet was subjected to heat treatment (i.e., annealing) only, again, the formability was not improved because the texture of crystal grains in the magnesium alloy sheet was not much changed.

On the contrary, the magnesium alloy sheet of the present invention demonstrated maximum 103% increased room temperature formability and maximum 98% increased warm formability, compared with the conventional sheet, under every experimental temperature condition.

Particularly, as the amount of compressive deformation in the magnesium alloy sheet increases, twin volume fraction was increased, indicating that the accommodatable deformation to the thickness direction of the magnesium alloy sheet was increased. Also, the dislocations generated in the magnesium alloy sheet were eliminated through annealing, indicating the formability of the magnesium alloy sheet was increased.

Therefore, the above results confirmed that the magnesium alloy sheet of the present invention could be used widely in the industry requiring energy efficiency improvement, owing to the increased warm and room temperature formability obtained by forming twins with compressive deformation and eliminating the dislocations generated by the compressive deformation with annealing.

EXPERIMENTAL EXAMPLE 2 Analysis of Formability of Magnesium Alloy Sheet According to Compressive Deformation and Heat Treatment 2

To analyze more precisely the causes of the improvement of formability of the magnesium alloy sheet of the present invention, electron back scattered diffraction (EBSD) was performed with the thick region of the center of the specimen, which is experienced the biggest deformation during forming, before and after the forming as shown in the load-displacement graph obtained from Erichsen test (FIG. 7 and FIG. 8). The results are shown in FIG. 9˜FIG. 13 (FIG. 7, FIGS. 9˜11: Example 2; FIG. 8, FIG. 12, FIG. 13: Comparative Example 1).

FIG. 9˜FIG. 11 illustrate the results of EBSD performed with the magnesium alloy sheet of Example 2 before forming (FIG. 9), with the magnesium alloy sheet formed to the thickness of 3 mm by Erichsen test (FIG. 10), and with the magnesium alloy sheet formed to the thickness of 4 mm (FIG. 11).

FIG. 12 and FIG. 13 illustrate the results of EBSD performed with the magnesium alloy sheet of Comparative Example 1 before forming (FIG. 12) and with the magnesium alloy sheet formed to the thickness of 3 mm (FIG. 13).

According to FIG. 7 and FIG. 8, the magnesium alloy sheet of the present invention displays improved formability by compressive deformation to the rolling direction and heat-treatment.

According to FIG. 9, twins were formed in a large scale in the magnesium alloy sheet before forming by compressive deformation to the rolling direction and FIG. 10 and FIG. 11 reveal that twins decreased as the forming processed.

According to FIG. 12, twins were not formed in the magnesium alloy sheet treated with neither compressive deformation nor heat-treatment. Referring to FIG. 13, small amount of contraction twins or {10-11}-{10-12} double twins were formed along the forming process by the compressive deformation to the thickness direction, which may cause fracture of the material.

Therefore, the above results indicate that the magnesium alloy sheet of the present invention is characterized by improved formability at room temperature as a result of compressive deformation to the rolling direction and annealing, because the deformations generated during forming are accommodated by annihilating {10-12} twins massively formed without increasing dislocations in the magnesium alloy sheet.

Those skilled in the art will appreciate that the conceptions and specific embodiments disclosed in the foregoing description may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present invention. Those skilled in the art will also appreciate that such equivalent embodiments do not depart from the spirit and scope of the invention as set forth in the appended Claims. 

We claim:
 1. A method for increasing formability of magnesium alloy sheet comprising the following steps: forming {10-12} twins in magnesium alloy sheet (step 1); and annealing the magnesium alloy sheet of step 1 (step 2), wherein the magnesium alloy sheet comprises {10-12} twins present after the annealing and reduced dislocations than before the annealing.
 2. The method for increasing formability of magnesium alloy sheet according to claim 1, wherein the forming of the {10-12} twins in step 1 causes texture to develop, in which basal planes are aligned perpendicular to the rolling direction.
 3. The method for increasing formability of magnesium alloy sheet according to claim 1, wherein the step 1 is performed by applying compressive deformation to the magnesium alloy sheet.
 4. The method for increasing formability of magnesium alloy sheet according to claim 3, wherein the compressive deformation is applied to the direction parallel to the rolling plane of the magnesium alloy sheet.
 5. The method for increasing formability of magnesium alloy sheet according to claim 3, wherein the amount of compressive deformation is 1˜15%.
 6. The method for increasing formability of magnesium alloy sheet according to claim 1, wherein the annealing is performed at 200˜550° C. for 10˜480 minutes.
 7. The method for increasing formability of magnesium alloy sheet according to claim 6, wherein the annealing is performed at 350˜500° C. for 60˜240 minutes.
 8. A magnesium alloy sheet with increased formability, which is prepared by the method according to claim 1 and comprises the {10-12} twins. 